Electronic Configuration

Electronic configuration

The systematic distribution of an electron in different shells of an atom is called electronic configuration.

To explain the arrangement of electrons in different shells, Bohr and Bury purposed a scheme as given below:

1. The maximum number of electrons in each shell is determined by 2n² rule, where n is the number of the shell.

For example:

For the K shell, the maximum number of electrons will be 2n² = 2.(1)² = 2 × 1 = 2

1. The maximum number of electrons in the outermost orbit is 8 and in second last orbit is 18.
2. It is not necessary for an orbit to be completed before another begins.

The energy level or shell nearer to the nucleus s called lower energy level and energy level way from the nucleus is called higher energy level.

Valence shell and Valence electrons:

The outermost orbit or shell of an atom is called valence shell and the number of electrons present in the valence shell of an atom is called valence electrons. They are far from the nucleus. The valence electrons determine the valency of an element. Valance electron takes part in the chemical reaction. The given table shows the electronic configuration and valence electron of some elements,

From valence electron, we get various information. Some of the information are given below,

1. It gives information about the combining capacity of the element.
2. The valence electron of an element gives information about the position of the element in a periodic table.
3. The number of shells determines the period to which the element falls in the periodic table.

Valency(combining capacity of an element):

Valency is defined as the combining capacity of element or radicle with the other element or radical to form a molecule or a compound. It is represented by numbers like1, 2, 3, 4, 5, and 6. The IUPAC has defined valency as, 'The maximum number of univalent atoms (originally hydrogen or chlorine atoms) that may combine with an atom of the element under consideration, or with a fragment, or for which an atom of this element can be substituted.'

Ways of calculating valencies of an element or a radical are listed below in points:

1. Hydrogen, oxygen, and chlorine are taken as the standard elements to determine the valency of an element or radical. The number of hydrogen atoms or chlorine atoms or a double number of oxygen atoms with which one atom of an element combines is known as its valency. If an element does not combine with hydrogen then in such condition, the valency is determined by comparing it with that of chlorine or oxygen.
   For example, The valency of nitrogen (N) in ammonia (NH3) IS 3 because 1 atom of C combines with 4 atoms of H.
   The valency of sodium (Na) in sodium chloride (NaCl) is 1 because 1 atom of Na combines with 1 atom of Cl.
2. Valence electron also determines the valency of an element. For example, the valence electron of Silicon is four. So, the valency of silicon is 4.
3. Valency is also determined by the number of electrons lost, gained or shared by the element during molecule formation. For example, a sodium atom loses an electron and chlorine gain one electron to form a compound. So, the valency of sodium and chlorine is 1.

Variable valency (Those elements which have more than one number of valency is called variable valency.)

We use Greek prefixes to describes different valancies like mono for one, di for two, tri for three, tetra for four, penta for five and hexa for six. For example, sulphur and magnesium are divalent as they have valency two. The name of elements with the lower valency ends with a suffix- ous and that with the higher valency ends with the suffix- ic.

Some elements have changeable combining capacity. When an element shows two or more than two valencies, then it is called variable valency. For example, copper shows valency 1 in cuprous chloride (CuCl) and valency 2 in cupric chloride (CuCl2).

Following are some of the elements who show variable valency properties: